1. Field of the Invention
The present invention relates to SRAM and a method for testing the SRAM.
2. Description of Related Art
FIG. 1 is a diagram showing configurations of a memory cell 100 and peripheral circuits in a general complementary metal-oxide semiconductor (CMOS) type SRAM. Referring to FIG. 1, the memory cell 100 include a positive feedback loop (flip-flop) composed of two CMOS inverters INV10, INV20, and two N-channel MOS transistors N110, N120 for controlling connections between bit line pair BL10, BV20 and the inverters INV 10, INV20 (hereinafter referred to as selection transistors N110, N120).
In detail, the two inverters INV10, INV20 are connected between a first power supply VDD and a second power supply (GND). An output of the inverter INV10 is connected to an input of the inverter INV20 through a node ND110. An output of the inverter INV20 is connected to an input of the inverter INV10 through a node ND120. The node ND110 is connected to the bit line BL10 through the selection transistor N110 whose gate is connected to a word line WL100. The node ND120 is connected to the bit line BL20 through the selection transistor N120 whose gate is connected to the word line WL100.
By the word line WL100 being activated, the selection transistors N110, N120 become in ON states, which connects electrically the node pair ND 110, 120 and the bit line pair BL10, BL20, respectively. Thereby, the node pair ND110, 120 holds a voltage supplied from the bit line pair BL10, BL20 (data write). Alternatively, the voltage held by the node pair ND110, 120 is inputted into an un-illustrated sense amplifier through the bit line pair BL10, BL20. The sense amplifier fixes a value of the data by comparing the inputted voltage and a threshold (data read). Thus, writing of the data in the memory cell 100 or reading of the data from the memory cell 100 is performed.
Moreover, before the writing and reading of the data, the bit line pair BL10, BL20 is precharged to a power supply voltage VDD by a precharge circuit 200. The precharge circuit 200 include P-channel MOS transistors P10, P20 (hereinafter referred to as precharge transistors P10, P20) for electrically connecting the first power supply VDD and the bit line pair BL10, BL20 in response to the precharge control signal PRB10 inputted into their gates.
There is a case where even when a read test is performed on the SRAM of this configuration, the memory cell that has a small Static Noise Margin (SNM) and a high fraction defective cannot be detected.
Operations of the read test according to the related technology and their problems will be explained with reference to FIG. 1 and FIG. 2. FIG. 2 is a timing chart showing the operations of the read test according to the related technology.
By time T1, “1” is written in the ND110, and “0” is written in the node ND120. At time T1, by the precharge control signal PRB10 shifting to a low level, the bit line pair BL10, BL20 is precharged to a high level. At time T2, the precharge control signal PRB10 shifts to the high level, and at the same time the word line WL100 is activated and the memory cell is selected. Thereby, a connection between the bit line pair and the first power supply VDD is cut off, and the data written in the memory cell 100 is read.
Between time T2 and time T3 when the word line WL100 is deactivated, a voltage of the node ND120 holding data “0” is pulled up by the bit line BL20 that was precharged to the high level. Here, when the memory cell 100 is normal, a voltage in the node ND120 rises only to a voltage lower than a logic threshold voltage of the inverter INV10. For this reason, the voltage of the node ND120 returns to the low level (data “0”) with discharge of the bit line BL20. In this case, the data being read by time T3 becomes the same value as the written data, and it is checked that the memory cell is normal. On the other hand, when the memory cell 100 has abnormality and the voltage in the node ND120 exceeds the logic threshold voltage of the inverter INV10, data different (inverted) from the data that was written in advance is read (not illustrated). In this case, the memory cell concerned is determined to be an abnormal memory cell.
However, there is a case where even when the memory cell 100 has the abnormality, the voltage in the node ND120 rises only to a voltage lower than the logic threshold voltage of the inverter INV10, and the written data is not inverted. This is because when the data is read, the bit line BT20 is discharged, and accordingly the electric charges of the bit line are released faster than the voltage elevation of the node ND120. Thus, there is a case where even when the memory cell 100 has the abnormality, the data whose value is the same as that of the written data is read and it is determined to be no error (normal).
At the times T3 to T5, operations of the times T1 to T3 are repeated.
As in the above, there is a case where even when the SNM is inspected by the read test by the related technology, the abnormality of the memory cell cannot be detected. A circuit for testing that solves such a problem is described, for example, in Anne Meixner, Jash Banik, “Weak Write Test Mode: An SRAM Cell Stability Design for Test Technique,” International test conference, 1997 (Nonpatent Document 1).
Operations of the read test using the circuit for testing described in Nonpatent Document 1 will be explained with reference to FIG. 3 and FIG. 4. FIG. 3 is a diagram showing configurations of the memory cell 100 and peripheral circuits (the precharge circuit 200, and a circuit 300 for testing) described in Nonpatent Document 1. FIG. 4 is a timing chart showing operations of the read test using the circuit 300 for testing described in Nonpatent Document 1.
From a test start time to time T2, the circuit 300 for testing performs the same operations as those of the related art shown in FIG. 2. At time T2, the precharge control signal PRB10 shifts to the high level, and at the same time the word line WL100 is activated and the memory cell is selected. In doing this, the control signal inputted into the circuit 300 for testing becomes the high level and the control signal WR1 becomes the low level, whereby a voltage of the bit line BL20 is pulled up to the high potential side by the first power supply VDD and a voltage of the bit line BL10 is pulled down to the low potential side by the GND.
By the voltage of the node N120 being pulled up and by the voltage of the node N110 being pulled down, the logic threshold voltage of the inverter INV10 is pulled down from an actual value, and a logic threshold voltage of the inverter INV20 is pulled up. That is, the data that has been held by the node pair N110, N120 with the circuit 300 for testing becomes a state of being easily inverted. For this reason, even in the case of the abnormal memory cell such that the data would not be inverted by the related technology, it is possible to detect the abnormality because the written data is inverted by a connection of the bit line BT20 being precharged. Here, the circuit 300 for testing is configured that the node voltage is pulled up or pulled down so that inversion of the data may not occur in the case of a normal memory cell. Therefore, according to the method described in Nonpatent Document 1, it becomes possible to detect the abnormal memory cell that has a small SNM and cannot be detected by the related technology.